A purge apparatus restricts fuel vapor, which is produced in a fuel tank, from diffusing into the atmosphere. In such a purge apparatus, fuel vapor is introduced from a fuel tank into a canister accommodating an adsorbent therein, so that the absorbent temporarily adsorbs the fuel vapor. The fuel vapor adsorbed into the adsorbent is desorbed from the adsorbent by negative pressure generated in an intake pipe, so that the fuel vapor is turned into mixture. The mixture is emitted, and purged into the intake pipe of an internal combustion engine through a purge passage, during an operation of the internal combustion engine.
When, in such a purge apparatus, any leaking hole exists in the passage for introducing fuel vapor into the intake pipe of the internal combustion engine, the canister, or the like, fuel vapor may be emitted to the atmosphere through the leaking hole. When a leaking hole exists in the purge apparatus, the leaking hole needs to be early detected.
In, for example, a leakage diagnosis apparatus disclosed in JP-A-2004-293438, pressure in the purge apparatus is detected when the pressure decreases or increases, thereby a leakage diagnosis is performed to evaluate whether a leaking hole exists in the purge apparatus on the basis of the pressure or the change in the pressure. In this structure, existence or nonexistence of the leaking hole is diagnosed by detecting the pressure in the purge apparatus. For example, when fuel shakes in the fuel tank or when fuel vapor in a large amount is produced in the fuel tank, the pressure in the purge apparatus is liable to change. In such a condition, in which the pressure is liable to change in the purge apparatus, it is difficult to accurately perform the leakage diagnosis. In the above leakage diagnosis apparatus, therefore, the leakage diagnosis is executed in an idling state where the pressure in the purge apparatus becomes stable, or after the engine is stopped. Immediately after the engine stop, however, fuel temperature becomes higher due to, for example, heat generated in a fuel pump provided in the fuel tank. Consequently, a large amount of fuel vapor is produced, and the pressure in the purge apparatus is not stabilized. Accordingly, the leakage diagnosis after the engine stop is executed upon lapse of a predetermined time period, which is required for stabilization of the pressure in the purge apparatus.
However, pressure may still fluctuate in the purge apparatus, even when the leakage diagnosis is executed upon the lapse of the predetermined time period, in which production of fuel vapor is assumed to be stabilized, since the engine stop. Specifically, for example, when highly volatile fuel is used, fuel vapor may increase in the purge apparatus by decreasing pressure in the purge apparatus due to performing the leakage diagnosis. When the leakage diagnosis is performed in such a condition, the pressure in the purge apparatus changes due to the production of fuel vapor, and hence, the leakage diagnosis cannot be precisely performed.
Apart from the case of using highly volatile fuel, the leakage diagnosis cannot be precisely performed in the following conditions. For example, when a vehicle is being transported or towed while the engine of the vehicle stops, fuel vapor is produced by shaking fuel. Alternatively, when altitude of the vehicle changes, fuel vapor may be further produced due to change in pressure.